In the mid-80s, my father and I used to go walking along Clifton beach in Karachi every evening. There was a regular group of walkers who I grew to recognise - though the only one I clearly remember now is the stout man who wore a T-shirt one size too small with the words "It's My Body You're After" emblazoned across it.

As we walked, my father told me that there used to be a time when the coarse, dark sand beneath our feet was fine and white, and the water clear blue. That was before Karachi became a major port and Clifton beach suffered the impact of proximity to the harbour (not to mention Karachi's ineffective sewage system which disposes of waste in ways we would all rather not think about). I used to look at the sand and try to imagine it white, try to imagine a magic formula that could reverse the spills and waste of cargo ships.

I mention this because it only seems fair to say that it has been a long time since Clifton beach has been a picture-perfect stretch of sand and sea. But of all Karachi's beaches it is the one most connected to the heart of the city, so it is the first place I go when I want to feel Karachi's pulse.

Those walks with my father stopped in the latter half of the 80s as violence spread through the city and even the beaches didn't seem safe; for some years thereafter, the beach was comparatively unused, with groups of men rather than families making up the bulk of its visitors. But in the past few years, as Karachi has started to limp out of its cyclical violence, Clifton beach has become more vibrant and festive than I recall even from my childhood. There are halogen lights along the sea wall now, attracting thousands of visitors well after the sun has gone down, and late at night you can go on camel rides, see snake-and-mongoose fights, buy snacks from roadside vendors, eat roasted corn-on-the-cob just off the flame. They are all here, all Karachi's ethnicities and economic levels and layers of conservatism (girls in jeans walk happily alongside women in burkas), and this is where you have to look to remind yourself that the mix doesn't have to be incendiary.

Since the Greek cargo ship Tasman Spirit broke in two last week, while transporting 67,000 tons of oil from Iran to Karachi, and spilled at least 10,000 tonnes of its cargo into the sea, Clifton beach and the area around it has been closed. More than 15km of the coast has been affected by the spill. The ironic shrug with which Karachiites greet most evidence of negligence and betrayal by the authorities is starkly absent from their responses to the disaster.

Just after the ship broke up, a friend of mine who worked for a time in Karachi at a mangrove conservation project emailed me to say: "Those bastards. Bloody Iftikhar bloody whoever minister of bloody whatever shouldn't go around pretending they did all that was possible to avert this." Her anger is shared by all Karachiites I have spoken to, and much of it stems from the fact that the 24-year-old, single-hull ship ran aground on July 27, and for more than two weeks the Karachi port trust insisted everything was under control, even as the layer of oil surrounding the vessel became increasingly dense and dead marine life started to float to shore. This insistence continued until the moment when the officials had to admit that the ship was about to break in two.

At the time of writing, more than 35,000 tonnes of oil remain on board and though we are told it is secure, no one is inclined to believe this, as the people who say so are the same ones who insist that the 10,000 tonnes already spilled pose no harm to human or marine life.

There is no word yet on the extent of ecological damage, but experts says it is potentially "a major disaster", particularly if a change in wind direction sends the oil drifting towards the mangroves. The fishing villages along the coast could see their primary source of income go, literally, belly-up. The long-term effects of toxins on residents along the coast is unknown.

So the devastation of Clifton beach itself is fairly low down the list of "disastrous consequences" of the spill - but as a metaphor for the failure of officialdom there can be nothing starker than thousands of Karachiites rushing to Clifton on August 14, Pakistan's independence day, to celebrate the birth of the nation, only to find paramilitary forces blocking the way and dead creatures thrown by black waters on to sludge that was once sand.